In recent years, along with the rapid development of advertisement industry, various billboards are booming. Especially, the billboards at the side of a highway may be used to intuitively show advertised items and thus have been widely recognized in the advertisement industry.
Usually, there are two ways to show the conventional billboards. In one way, a printed poster is directly pasted onto an advertising wall, and in another way, a display device is hung on the advertising wall. However, for the former, usually the advertising wall at the side of the highway is arranged on a high pillar, and the poster needs to be pasted onto the advertising wall manually, so it is very inconvenient to replace the poster. For the latter, usually an image is directly displayed by a conventional display device by means of a light-emitting source, and thus a very high light intensity of the display device is demanded when it is used to display the poster outdoor during the daytime. Otherwise, in the strong sun shine, it is very difficult for human eyes to clearly view the image, i.e., a visual effect is unsatisfactory.